Dynamic Wetting of Solid Surfaces, Influence of Surface Structures and Surface Active Polymers

Abstract: The effect of surface structures on the dynamics of wetting was investigated through measurements of the dynamic contact angle and spreading velocity of oil droplets spreading over surfaces with parallel and gridlike V-shaped channels. A rim of liquid was observed to spread out in the channels ahead of the dropfront after a short time of spreading (seconds). The characteristics of the rim were investigated as well, with respect to channel depth and spacing and compared to the spreading characteristics in channels with no intersections, i.e. parallel channels. The macroscopic droplet appears to be dependent on the rim for spreading, before the rim had appeared the dropfront spread stepwise between the perpendicular channels and did not advance until the channel was liquid filled. The spreading on the filled channels was correlated to the surface area covered by channels rather than the depth and width of the individual channel. The spreading of the rim follows the expected scaling with (time)1/2 and (channel depth)1/2. We report on the wetting of silica by aqueous solutions of triblock copoly(ethylene oxide-tetrahydrofuran-ethylene oxide), PEO-PTHF-PEO. The wetting behaviour was measured by means of a Wilhelmy force balance and by direct images of the contact angle. The results show that the three-phase contact line (tcl) advances in jumps over the surface when this is immersed with constant rate into copolymer solutions. The wetting results are compared with adsorption data obtained by ellipsometry and dynamic surface tension measurements. It is clear that the stick-slip spreading behaviour results from the same basic mechanisms as have previously been proposed to be responsible for the stick-slip spreading observed for short chain cationic surfactants. The pinning of the contact line is due to the formation (through adsorption) of an autophobic copolymer layer at the solid-vapour interface.